> -----Original Message----- > From: Alexander Graf [mailto:agraf@xxxxxxx] > Sent: Monday, October 31, 2011 12:21 AM > To: Bhushan Bharat-R65777 > Cc: Liu Yu-B13201; kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx > Subject: Re: [PATCH v2] Fix DEC truncation for greater than > 0xffff_ffff/1000 > > > On 20.10.2011, at 09:18, Bhushan Bharat-R65777 wrote: > > > > > > >> -----Original Message----- > >> From: Liu Yu-B13201 > >> Sent: Thursday, October 20, 2011 12:35 PM > >> To: Bhushan Bharat-R65777; agraf@xxxxxxx > >> Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx > >> Subject: RE: [PATCH v2] Fix DEC truncation for greater than > >> 0xffff_ffff/1000 > >> > >> > >> > >>> -----Original Message----- > >>> From: Bhushan Bharat-R65777 > >>> Sent: Thursday, October 20, 2011 2:41 PM > >>> To: Liu Yu-B13201; agraf@xxxxxxx > >>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx > >>> Subject: RE: [PATCH v2] Fix DEC truncation for greater than > >>> 0xffff_ffff/1000 > >>> > >>> > >>> > >>>> -----Original Message----- > >>>> From: Liu Yu-B13201 > >>>> Sent: Wednesday, October 19, 2011 4:28 PM > >>>> To: Bhushan Bharat-R65777; agraf@xxxxxxx > >>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx; > >>> Bhushan Bharat- > >>>> R65777 > >>>> Subject: RE: [PATCH v2] Fix DEC truncation for greater than > >>>> 0xffff_ffff/1000 > >>>> > >>>> > >>>> > >>>>> -----Original Message----- > >>>>> From: kvm-ppc-owner@xxxxxxxxxxxxxxx > >>>>> [mailto:kvm-ppc-owner@xxxxxxxxxxxxxxx] On Behalf Of > Bharat Bhushan > >>>>> Sent: Wednesday, October 19, 2011 12:16 PM > >>>>> To: agraf@xxxxxxx > >>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; bharatb.yadav@xxxxxxxxx; Bhushan > >>>>> Bharat-R65777 > >>>>> Subject: [PATCH v2] Fix DEC truncation for greater than > >>>>> 0xffff_ffff/1000 > >>>>> > >>>>> kvmppc_emulate_dec() uses dec_nsec of type unsigned > long and does > >>>>> below calculation: > >>>>> > >>>>> dec_nsec = vcpu->arch.dec; > >>>>> dec_nsec *= 1000; > >>>>> This will truncate if DEC value "vcpu->arch.dec" is greater than > >>>>> 0xffff_ffff/1000. > >>>>> For example : For tb_ticks_per_usec = 4a, we can not set > >>> decrementer > >>>>> more than ~58ms. > >>>>> > >>>>> Signed-off-by: Bharat Bhushan <bharat.bhushan@xxxxxxxxxxxxx> > >>>>> --- > >>>>> arch/powerpc/kvm/emulate.c | 12 +++++++----- > >>>>> 1 files changed, 7 insertions(+), 5 deletions(-) > >>>>> > >>>>> diff --git a/arch/powerpc/kvm/emulate.c > >>> b/arch/powerpc/kvm/emulate.c > >>>>> index 8af3bad..e7f3da4 100644 > >>>>> --- a/arch/powerpc/kvm/emulate.c > >>>>> +++ b/arch/powerpc/kvm/emulate.c > >>>>> @@ -84,6 +84,7 @@ static bool kvmppc_dec_enabled(struct kvm_vcpu > >>>>> *vcpu) void kvmppc_emulate_dec(struct kvm_vcpu *vcpu) { > >>>>> unsigned long dec_nsec; > >>>>> + unsigned long long dec_time; > >>>>> > >>>>> pr_debug("mtDEC: %x\n", vcpu->arch.dec); #ifdef > >>>>> CONFIG_PPC_BOOK3S @@ -103,11 +104,12 @@ void > >>>>> kvmppc_emulate_dec(struct > >>> kvm_vcpu *vcpu) > >>>>> * host ticks. */ > >>>>> > >>>>> hrtimer_try_to_cancel(&vcpu->arch.dec_timer); > >>>>> - dec_nsec = vcpu->arch.dec; > >>>>> - dec_nsec *= 1000; > >>>>> - dec_nsec /= tb_ticks_per_usec; > >>>>> - hrtimer_start(&vcpu->arch.dec_timer, > >>>>> ktime_set(0, dec_nsec), > >>>>> - HRTIMER_MODE_REL); > >>>>> + dec_time = vcpu->arch.dec; > >>>>> + dec_time *= 1000; > >>>>> + do_div(dec_time, tb_ticks_per_usec); > >>>>> + dec_nsec = do_div(dec_time, NSEC_PER_SEC); > >>>>> + hrtimer_start(&vcpu->arch.dec_timer, > >>>>> + ktime_set(dec_time, dec_nsec), > >>>>> HRTIMER_MODE_REL); > >>>>> vcpu->arch.dec_jiffies = get_tb(); > >>>>> } else { > >>>>> hrtimer_try_to_cancel(&vcpu->arch.dec_timer); > >>>>> -- > >>>>> 1.7.0.4 > >>>>> > >>>> > >>>> How does this impact performance? > >>>> 64bits multiplication and division looks slow. > >>>> > >>> > >>> I tried running below test as guest, with and without > this patch and > >>> tried to find latency added by this patch. Also I run > this for a list > >>> of timeouts (1, 2 , 4, 8, 16, 32ms) one by one. > >>> > >>> - get TB (say a). > >>> - set decrementer in auto reload mode. > >>> - wait for 1000 timebase interrupts. > >>> - Get timebase delta (b = get_tb - a). > >>> > >>> (b1 - b2) <=> b1 with this patch and b2 > >>> without this patch. And roughly I found any impact. For example: > >>> For 1ms = ( 48a19d8 - 48a1459) = 0x57f = .0018% For 32ms = > >>> (90fdfa23 - 90fdfe79) = -(0x456) > >> > >> Doesn't (b1 - b2) mean difference of the last one > interrupt between have > >> patch and havenot patch? > >> The time of previous 999 interrupts is hidden in the cpu idle time. > >> > > > > > > Probably I have not described properly. b1 and b2 are > delta, not timestamp. In this case I run this test with patch > > Print on console the total time (in TB tick) for which > this test runs. Which includes time of all 1000 interrupts. > > > > Then I exit and rerun the above test case without patch > > Then mannualy calculated difference/percentage etc. > > > > Also if you see timebase delta, it suggest that it is not > timebase difference of one decrementer. > > So Yu are you ok with this patch? If so, please ack. > Acked-by: Liu Yu <yu.liu@xxxxxxxxxxxxx> Thanks, Yu -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
